This invention relates to a novel method for inspecting glass viewing windows for cathode-ray tubes to detect objectionable cords; and particularly to such a method as would permit those viewing windows with objectionable cords to be identified at the glass factory or at incoming parts inspection at the tube-making factory. "Cords" are minor variations of the index of refraction in localized regions of the window.
A cathode-ray tube, such as a television picture tube, usually comprises a glass window and a luminescent viewing screen supported on the inner surface of the window. Video images are generated on the screen as an array of video image elements and are viewed through the window. The video image elements are small, light-emitting areas of the screen of similar size which are arranged in a repetitive array. There are normally hundreds of thousands of such elements on the screen. In positive-tolerance, shadow mask tubes, the elements are defined by the size and shape of the electron beam spots impinging on the screen. In negative-tolerance, shadow mask tubes, the elements are defined by the size and shape of the holes in the light-absorbing matrix between the viewing screen and the glass window. The inner surface of the window is stippled for reasons related to the fabrication of the window and/or of the screen. The stippled finish has the effect of scattering light which passes through it.
As fabricated, glass viewing windows for cathode-ray tubes have cords present to a greater or lesser degree. Cords are within the thickness of the glass and can occur at different distances from the inner surface of the window. Cords usually result from minor variations in glass composition, although other causes are possible. Cords are transparent and do not produce a change in color or opacity where they occur. However, the minor variations in index of refraction which are characteristic of cords may cause the viewed video image to be distorted at localized regions of the window. These distortions are particularly objectionable where the change in index of refraction is relatively sharp, relatively large, and the boundary of the cord parallels or nearly parallels the alignment of the video image elements of the viewed image. In many cases, as where the boundary of the cord crosses the alignment of the image elements at greater angles than near parallel, the presence of cords is not objectionable, even where the transition is large and/or sharp. The alignment of image elements as used herein refers to the visual effect of orderly rows of elements in a specific direction or directions in a repetitive array of elements.
In some prior methods of inspecting viewing windows for cords, a white background is pressed against the inner surface of the window, bright light is projected through the window onto the white background and the inspector observes the background through the window looking for variations in brightness caused by cords, called "shadows," which are cast on the background. The inspector moves his eyes to different positions and angles with respect to the glass surface, hoping to find the cords. Some inspectors become quite skilled at this. But, even under the best of conditions and with superior skill, cords are difficult to detect because the glass is transparent at and around the boundary of a cord, where there is only a transition of the refractive indices. The task is made more difficult due to the presence of the light-scattering finish on the inner surface of the window, which blurs the "shadows" caused by the cords; and due to the usual presence of a gray tint in the glass to reduce the light transmission through the window, which makes the "shadows" more difficult to see.